In oil, gas, and geothermal drilling operations, a drill bit attached to a drill stem is rotated to drill a wellbore through subsurface geologic formations. Roller-cone drill bits usually comprise a plurality of legs having a rotatable cone attached by a bearing to the spindle of each leg. Other types of drill bits such as drag-type bits do not use bearings or other moving components. As a drill bit is rotated, drilling fluid is circulated to cool the drill bit and to transport rock cuttings from the wellbore. The drilling fluid is pumped down through the drill stem, through ports in the drill bit, and up through the annulus between the drill stem and the wellbore.
A drill bit will wear as it is rotated to advance the depth of the wellbore. The length of time that a drill bit can be used before it becomes excessively worn depends on a variety of factors such as the hardness and composition of the rock and the drill stem weight that the operator places on the drill bit. The drill bit should be replaced when its rate of penetration has diminished to an unacceptable level or when torque values in rotating the drill string exceed an acceptable limit. An operator can measure the rate of penetration and the torque values from the surface.
Other factors which normally require the replacement of a drill bit cannot be measured from the surface. For example, a roller-cone drill bit should be replaced when the bit bearings are excessively worn or when the wellbore is being drilled undergauge. As the drill bit is rotated, the load-bearing surfaces between a cone and the spindle of a leg will begin to wear. As the surfaces wear, the cone will begin to rotate eccentrically about the spindle until the cone seizes, becomes excessively worn, or is separated from the spindle. In a sealed bit, the bearing will begin to fail after the seal between the cone and the spindle is damaged. If a bit bearing should fail and leave a cone in the wellbore, drilling operations are usually discontinued until the cone is "fished" from the wellbore.
Loss of borehole gauge of a roller-cone bit is due to bearing wear or to abrasion of the gauge-maintaining portion of the drill bit cones against the wellbore wall. In a drag-type bit, loss of borehole gauge is due to wear of the gauge maintaining cutters. Loss of gauge is undesirable because there is a greater possibility of differential pressure sticking between the drill string and the wellbore. Loss of gauge is especially undesirable in specialized drilling operations such as in highly deviated wells because the operator may have difficulty in maintaining directional control of the wellbore. Although loss of gauge can be reduced by hard-facing certain portions of the bit, loss of gauge remains a problem in drilling operations.
To avoid the cost of retrieving lost cones from the wellbore, most drill bits will generally be tripped out of the wellbore and replaced before the bit bearings fail. Because each drill bit is not used to the extent of its maximum useful life, this practice is costly because more drill bits are required to drill the wellbore to a particular depth. The practice of pulling drill bits "green" is particularly costly because the drill pipe and drill collars must be tripped each time that a drill bit is replaced. In deep drilling operations or in offshore drilling operations which may cost up to $130,000 U.S. per day, an operator should maximize drilling time by using each drill bit to the full extent of its useful life.
Although techniques have been proposed to detect bearing failure or loss of borehole gauge of a drill bit, the techniques are not acceptable for commercial use because the dependability of the techniques in a drilling mud environment is unproven. Therefore, a need exists for a method and apparatus to detect excessive wear of the wear surfaces in a drill bit.